AUSTRALIAN Nick Kyrgios has apologised and been fined after making derogatory comments about Stan Wawrinka's girlfriend during their Rogers Cup match.

Nick Kyrgios was dubbed the 'bad boy' of tennis when he was fined for his behaviour during Wimbledon

French Open champion Wawrinka called the behaviour of Kyrgios "unacceptable" after the comments in Wednesday night's second-round contest in Montreal.

On-court microphones recorded Kyrgios, 20, walking to the net and saying: "Kokkinakis ****** your girlfriend. Sorry to tell you that mate" - an reference to his Australian Davis Cup team-mate Thanasi Kokkinakis.

Teenage Croatian player Donna Vekic, 19, has been linked to Wawrinka since he divorced his wife Ilham in April.

Kyrgios has been hit with an initial fine of £6,405 but that could rise, said a spokesman for men's tour the ATP, who added: "The ATP also said it is still reviewing that matter and additional penalties may be forthcoming."

Speaking on court after the exchange, Kyrgios claimed he had been provoked, saying: "He [Wawrinka] was getting a bit lippy with me. Kind of in the heat of the moment, I don't know. I just said it."

After sleeping on the issue, however, he then issued an apology last night via Facebook, on which he said: "I would like to take this opportunity to apologise for the comments I made during the match last night vs Stan Wawrinka.

"My comments were made in the heat of the moment and were unacceptable on many levels.

"In addition to the private apology I've made, I would like to make a public apology as well. I take full responsibility for my actions and regret what happened."

After their match, Wawrinka had tweeted: "So disappointing to see a fellow athlete and colleague be so disrespectful in a way I could never even imagine.

"What was said I wouldn't say to my worst enemy. To stoop so low is not only unacceptable but also beyond belief.

"There is no need for this kind of behaviour on or off the court and I hope the governing body of this sport does not stand for this and stands up for the integrity of this sport that we have worked so hard to build."

Controversy has followed Kyrgios around since his 2014 breakthrough when he knocked Rafael Nadal out of Wimbledon.

At this year's Wimbledon he was moved to deny accusations of abusing an umpire and throwing a game and, while on Davis Cup duty with his country against Kazakhstan, he shouted "I don't want to be here" while on court.

Even world No1 Novak Djokovic felt compelled to have his say. "In my opinion it was completely unnecessary to comment something like that," he said. "I think he's going to learn a lesson in a hard way. Hopefully this won't happen to him anymore. I understand that every player goes through a certain tantrums, emotional ups and downs during matches but there's no excuse for directing your tantrums to your opponent - and especially to somebody that's not even there."

Andy Murray, meanwhile, who could face Kyrgios in September's Davis Cup semi-final against Australia, continued his progress towards a potential semi-final with Nadal after dispatching big serving left-hander Gilles Muller 6-3, 6-2 in just over an hour.

"It's extremely humid here, conditions are tricky,"said Murray said. "But I feel I'm returning a little better this week and building the confidence again on the hard courts."